Sunday, July 28, 2013

It's been a pretty long time since I've written anything about stellar evolution, but a few weeks ago, a paper on arXiv caught my eye regarding the identification of the progenitor of a recent supernova. Supernovae are one of those events that get everyone excited. With ever advancing technology, astronomers are discovering hundreds of them a year, but they are such rare events for any given galaxy, that we very rarely get to study them in any great detail beyond perhaps their light curve or a bit of spectroscopy.

The best example we have where we've been able to study a supernova up close an personal was the well publicized SN 1987a. With this, astronomers were not only able to study the morphology of the resulting debris, but sufficiently high resolution images of the region were available to pin down the progenitor star. While '87a was a bit of an oddball in that it came from a blue supergiant which were not thought to be supernovae, it did confirm that at least this type II supernova come from a high mass star, an important bit of observational evidence.

It's been 26 years since that historical supernova, and while it still holds the record for the nearest supernova to us since the development of the telescope making it one of the best for study, our telescopes have improved greatly since then. In particular, the Hubble Space Telescope was launched in 1990 giving an unparalleled view of galaxies even further out. So with this advance in technology, have we pinned down the progenitors of other core collapse supernovae?

Most certainly. To date, over a dozen additional supernovae have progenitors that have been identified. So what have we learned?

For the most part, the notion that type II supernovae should come from red and yellow supergiant stars has held up exceptionally well. Of the 13 supernovae for which progenitors have been identified since '87a, 12 of them have had progenitors identified as red or yellow supergiants. Only one, SN 2005gj, stands out as an exception. This star appears to be Luminous Blue Variable (LBV) much like SN 1987a was. However, this supernova defies the typical type II classification. One of the reasons astronomers give a type II designation to supernovae is when they see hydrogen emission lines in the supernova's spectra, indicating the star still has its atmosphere as opposed to being a stripped down white dwarf that is pushed over its Chandrashekar limit by having mass dumped on it from a partner. While the presence of hydrogen in the spectra was the case for 2005gj, it also exhibited all the telltale signs of a type Ia supernova. As such, the interpretation is that 2005gj was a type IIa supernova surrounded by a very dense cloud of hydrogen, perhaps of its own making. In other words, the star may have been pushed over the limit, becoming a supernova, before it completely shedded its outer layers as a planetary nebula.

Confirming that type II supernovae come from these massive, highly evolved stars is a powerful test of our understanding of stars. In particular, the mechanics of what drives a supernova are all buried deep within the star's core, a place that is quite difficult to probe to confirm theoretical models. While there are still many questions to be answered (such as the amazing diversity of energies of these events) the core predictions of models made decades before we ever identified a single supernova progenitor, that core collapse supernovae come from highly evolved, massive stars, has been confirmed in over a dozen cases.

For those interested, I've put some information below on articles related to identifying these progenitors.